Precious metals and alloys of the precious metals, such as Pt, Pt alloys, Ir and Ir alloys are used in ignition plug electrodes (center electrodes and earth electrodes) to be used in internal combustion engines. Hitherto, in use of these precious metal materials as electrodes, a chip-shaped precious metal material has been commonly bonded to a base material including a heat-resistant material such as a Ni alloy on the ignition plug side. As a method for bonding the precious metal chip, laser welding is known. In the laser welding, the outer peripheral surface of a part at which a precious metal chip and a base material are in contact with each other is irradiated with a laser beam to melt the irradiated part, so that the precious metal chip and the base material are bonded to each other.
Laser welding melts the outer peripheral surface of a bonded part between a precious metal chip and a base material, but a part inside the bonded surface may be unmelted. Here, a void may remain at the bonded interface, and when the temperature is elevated during use, a gas in the void is expanded, so that cracks tend to be generated, leading to separation of the precious metal chip from the base material.
In laser welding, a molten zone is generated on the outer peripheral surface of a bonded part. The molten zone is a region formed by melting/solidification of a material. The molten zone is different in composition of the material and structures from a precious metal parent material, and is fragile and poor in electrical properties. In other words, the molten zone is not a portion that is effective as a plug material. Thus, the precious metal chip is required to have an excessive length, which is not preferable from the viewpoint of a cost based on the use amount of a precious metal, or resource saving.
The present inventors recommend application of an electrode chip formed from a precious metal layer and a substrate and having a clad structure in place of a conventional metal chip including a precious metal. The electrode chip for a plug electrode is formed by bonding a thin precious metal layer to a substrate including, for example, Ni or a Ni alloy by diffusive bonding under controlled bonding conditions. The electrode chip contributes to reduction of the use amount of a precious metal because a precious metal layer and a substrate are bonded to each other with optimum bonding conditions determined beforehand, so that the thickness of the precious metal layer can be minimized while the electrode characteristics and life are secured. Further, bonding for mounting of a chip can be facilitated because a Ni alloy etc. that is identical to a base material for the plug electrode is used as the substrate.